Library of velocity dependent extended spread-spectrum codes

ABSTRACT

A method and apparatus is provided for determining a propagation time delay. The apparatus includes at least one source adapted to generate a plurality of positioning signals, at least one receiver deployed along a seismic sensing cable, wherein the receiver is adapted to receive the plurality of positioning signals from the at least one source, and a plurality of computed Doppler-shifted positioning signals corresponding to the plurality of positioning signals. The apparatus also includes a signal processing unit adapted to determine a propagation time delay between the source and the receiver using the generated positioning signals, the received positioning signals, and the plurality of computed Doppler shifted positioning signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to acoustic ranging, and, moreparticularly, to acoustic ranging using velocity dependent extendedspread-spectrum codes.

2. Description of the Related Art

Underwater seismic exploration is widely used to locate and/or surveysubterranean geological formations for hydrocarbon deposits. A surveytypically involves deploying one or more seismic sources and one or moreseismic sensors at predetermined locations. For example, a seismic cableincluding an array of seismic sensors may be deployed on the sea floorand a seismic source may be towed along the ocean's surface by a surveyvessel. The seismic sources generate acoustic waves that travel to thegeological formations, where they are reflected and propagate back tothe seismic sensors. The seismic sensors receive the reflected waves,which are then processed to generate seismic data. Analysis of theseismic data may indicate probable locations of geological formationsand hydrocarbon deposits.

The accuracy of the seismic analysis may be limited by uncertainties inthe seismic source and sensor positions. The positions of deployedseismic sources and seismic sensors may be estimated using modellingtechniques that predict the position of the deployed seismic sources.For example, the position of a seismic cable on the sea floor may beestimated using models that consider the physical characteristics of theseismic cable (e.g., weight, diameter, etc.) and the effect of predictedsea currents on the seismic cable as it descends to the sea floor.However, such methods are predicated on a limited knowledge of theproperties of water in the catenary, as well as the geology of the seafloor, and thus they only provide an estimate of the seismic cable'slocation.

A variety of measurement techniques have been developed to determine theposition of the seismic sources and the seismic sensors as the seismicsensors descend through the catenary and come to rest on the sea floor.One such technique is time delay estimation, which determines thepositions of arrays of seismic sources and seismic sensors by measuringthe time it takes for a signal, such as a chirp, to travel between theseismic sources and seismic sensors. For example, an acoustic source maybe deployed on a buoy at the sea surface. One or more receivers may bedeployed along a seismic cable resting on the sea floor. The distancebetween the acoustic source and the receivers, and, consequently, theposition of the seismic cable, may be determined by cross-correlating apositioning signal emitted by the acoustic source with the positioningsignal received by the receivers. The cross-correlation produces a peakin the cross-correlation estimate that corresponds to a time lag causedby propagation of the positioning signal from the acoustic source to thereceivers.

The ocean's surface, however, is not an ideal platform for the acousticsources and/or receivers that are used in time delay estimation.Movement of the acoustic source and/or receiver as it rides the roughsea surface may introduce Doppler shifts into the positioning signal,which may degrade the cross-correlation estimates. Even moderately heavyseas with a significant wave height (SWH) of about 8 meters mayaccelerate a buoy or vessel to velocities of about 2-3 meters persecond. The resulting Doppler shift may destroy the peak in thecross-correlation estimate in up to 60% of the attempted measurements.Similarly, the motion of the seismic cable may degrade thecross-correlation estimates, making it difficult to determine thelocation of the seismic cable as it descends through the catenary to thesea floor. For example, FIG. 1 shows a model cross correlation estimate10 calculated with stationary sources and sensors. A peak 20 is evidentat a time lag of zero. A second correlation estimate 30 at various timelags is calculated including a Doppler shift, and a peak 40 is evidentat a non-zero time lag. The amplitude of the peak 40 is reduced relativeto the peak 20 because of the Doppler shift.

SUMMARY OF THE INVENTION

In one aspect of the instant invention, an apparatus is provided fordetermining a propagation time delay. The apparatus includes at leastone source adapted to generate a plurality of positioning signals, atleast one receiver deployed along a seismic sensing cable, wherein thereceiver is adapted to receive the plurality of positioning signals fromthe at least one source, and a plurality of computed Doppler-shiftedpositioning signals corresponding to the plurality of positioningsignals. The apparatus also includes a signal processing unit adapted todetermine a propagation time delay between the source and the receiverusing the generated positioning signals, the received positioningsignals, and the plurality of computed Doppler shifted positioningsignals.

In another aspect of the present invention, a method is provided fordetermining a propagation time delay. The method includes generating atleast one positioning signal using at least one source, receiving the atleast one positioning signal with at least one receiver positioned alonga seismic cable, and providing at least one computed Doppler-shiftedpositioning signal corresponding to the at least one positioning signal.The method also includes determining at least one propagation time delayfrom the source to the receiver using the generated positioning signal,the received positioning signal, and the at least one computedDoppler-shifted positioning signal.

In yet another aspect of the present invention, a method is provided forforming a library. The method includes determining a plurality ofvelocities, selecting a plurality of positioning signals, anddetermining a plurality of computed Doppler-shifted positioning signalsfor each of the plurality of positioning signals using the plurality ofvelocities. The method also includes providing an index to the pluralityof computed Doppler-shifted positioning signals.

In a further aspect of the present invention, a library including a datastructure encoded on a computer-readable storage medium is provided. Thelibrary includes a plurality of Doppler-shifted positioning signalsformed by determining a plurality of velocities, selecting a pluralityof positioning signals, and determining a plurality of computedDoppler-shifted positioning signals for each of the plurality ofpositioning signals using the plurality of velocities. The library alsoincludes an index of the plurality of computed Doppler-shiftedpositioning signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIG. 1 shows a prior art correlation estimate;

FIGS. 2A-D each show aspects of first and second exemplary systems foracoustic ranging, in accordance with alternate embodiments of thepresent invention;

FIG. 3 shows an illustrative example of a positioning signal that may beused by the systems shown in FIGS. 2A-D, in accordance with oneembodiment of the present invention;

FIG. 4 shows a method of acoustic ranging, in accordance with oneembodiment of the present invention;

FIG. 5 shows a first method of correlating that may be used as a part ofthe method shown in FIG. 4;

FIG. 6 shows a library that may be used by the first method ofcorrelating shown in FIG. 5;

FIG. 7 shows a second method of correlating that may be used as a partof the method shown in FIG. 4, in accordance with one embodiment of thepresent invention; and

FIG. 8 shows a rack mounted computer system that may be used in thefirst and second exemplary systems for acoustic ranging.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Referring now to FIGS. 2A-B, a first exemplary system 100 fordetermining a propagation time delay is shown, in accordance with afirst embodiment of the present invention. In the illustratedembodiment, the first exemplary system 100 includes, but is not limitedto, a survey vessel 105, a source 110, and a seismic cable 115. Thesurvey vessel 105 may be deployed on a surface 116 of a body of water117, which, in alternative embodiments, may be freshwater, sea water, orbrackish water. Similarly, the source 110 may be deployed in anydesirable manner at or near the surface 116 of the body of water 117.For example, the source 110 may be mounted on the survey vessel 105,suspended beneath a buoy, towed behind a second vessel, or deployed inany like manner. It will further be appreciated that more than onesource 110 may be deployed without departing from the scope of thepresent invention. In one embodiment, a velocity meter 122 may becoupled to the source 110, although this is not necessary for thepractice of the present invention.

The seismic cable 115 includes one or more sensors 125. The sensors 125receive a variety of signals, including, but not limited to positioningsignals 130, seismic signals (not shown), and the like. In particular,the sensors 125 are designed to receive at least the positioning signals130 generated by the source 110. In one embodiment, the sensors 125 mayalso receive reflected seismic signals (not shown) that may be analysedto locate and/or survey geologic formations such as hydrocarbondeposits. Velocity meters 122 may also be coupled to the sensors 125,although this is not necessary for the practice of the presentinvention. The seismic cable 115 may be deployed from the survey vessel105 by any of a variety of means well-known to those of ordinary skillin the art, including, but not limited to, spooling the seismic cable115 from the stem of the survey vessel 105 as the survey vessel 105moves across the surface 116 of the body of water 117. In oneembodiment, as the seismic cable 115 is being deployed, the seismiccable 115 descends through the catenary until it reaches the floor 140of the body of water 117.

The size and shape of the seismic cable 115, currents in the body ofwater 117, and other like factors may influence the path the seismiccable 115 takes on its descent through the catenary. For example, and asillustrated by the arrows in FIG. 2A, portions of the seismic cable 115may descend at different speeds and in different directions. AlthoughFIG. 2A only shows the vertical variations in the rate of descent andposition of the seismic cable 115, it will be appreciated that an actualseismic cable 115 may also move horizontally and that the horizontalmotion and position of the seismic cable 115 may vary along the lengthof the seismic cable 115. Similarly, irregularities in a floor 140 underthe body of water 117, such as a bump 145 shown in FIGS. 2A-B, mayinfluence the position of the seismic cable 115 as it rests upon thefloor 140.

The conditions in the body of water 117, as well as the geometry of thefloor 140, may not be known in advance. These unknown and/or unforeseenconditions may determine the path of the seismic cable 115 as itdescends through the catenary and the position of the seismic cable 115as it rests upon the floor 140. Consequently, it may not be possible topredict the exact location of the seismic cable 115 during and afterdeployment. The accuracy of seismic surveys may, however, depend upon anaccurate knowledge of the location of the seismic cable 115.

To determine the position of the seismic cable 115, the source 110generates at least one positioning signal 130 that is received by theone or more sensors 125 positioned along the seismic cable 115.Virtually any positioning signal 130 now known or known in the future tothe art may be used. In one embodiment, the generated positioning signal130 is a spread-spectrum sequence. For example, the generatedpositioning signal 130 may be one of an orthogonal set of sequences,such as a Kasami sequence, a Maximal sequence, and the like. It will beappreciated that sequences are sometimes referred to as codes. As usedhereinafter, the term “sequence” will be understood to refer tosequences, codes, and the like.

The received positioning signal and the generated positioning signal 130may then be communicated to a signal processing unit 150. For example,in one embodiment, the source 110 and/or the sensors 125 may communicatethe received positioning signal and the generated positioning signal 130to the signal processing unit 150 via a data telemetry unit (not shown)included in the source 110 and/or the sensors 125. However, inalternative embodiments, the received positioning signal and thegenerated positioning signal 130 may be communicated to the signalprocessing unit 150 by any desirable means such as radio-frequencytransmissions, optical devices, and the like.

The signal processing unit 150 may correlate the received positioningsignal and the generated positioning signal 130 in a conventional mannerwell-known to those of ordinary skill in the art having benefit of thepresent disclosure. For example, as discussed in more detail below, thesignal processing unit 150 may form a cross-correlation estimate bycross-correlating the received positioning signal and the generatedpositioning signal 130. A peak in the cross-correlation estimatecorresponding to a propagation time lag may be used to determine thelocation of the seismic cable 115. Although the signal processing unit150 depicted in FIGS. 2A-B is positioned on the vessel 105, the presentinvention is not so limited. In alternative embodiments, portions of thesignal processing unit 150 may be positioned in the seismic cable 115,on the source 110, on the survey vessel 105, or at any other desirablelocation without departing from the scope of the present invention.

As described in more detail below, computed Doppler-shifted positioningsignals (not shown) corresponding to the positioning signals 130 arealso provided, in accordance with the first embodiment of the presentinvention. The computed Doppler-shifted positioning signals may then becorrelated with the received positioning signal and/or the generatedpositioning signal 130 to determine the position of the sensors 125 onthe seismic cable 115. Consequently, the present invention may be usedfor determining a propagation time delay between the source 110 and thesensors 125. Thus, it may be possible to determine the position of theseismic cable 115 in situations wherein movement of the source 110, thesurvey vessel 105, the cable 115, and/or the sensors 125 Doppler shiftsthe frequencies of the received signal and/or the positioning signal130.

Referring now to FIGS. 2C-D, a second exemplary system 200 fordetermining a propagation time delay, in accordance with a secondembodiment of the present invention, is shown. The survey vessel 105 inthe second exemplary system 200 may, in one embodiment, deploy aplurality of streamer cables 205 at or near the surface 116 of a body ofwater 117, which, in alternative embodiments, may be freshwater, seawater, or brackish water. In alternative embodiments, the streamercables 205 may be deployed below the surface of the body of water 117.The streamer cables 205 include a plurality of sensors 125. In oneembodiment, velocity meters 122 are coupled to the sensors 125. Althoughtwo streamer cables 205 are shown in FIGS. 2C-D, the present inventionis not so limited. In alternative embodiments, more or fewer streamercables 205 may be deployed without departing from the scope of thepresent invention.

One or more transceivers 210 are coupled to the streamer cables 205. Aswith the first embodiment illustrated in FIGS. 2A-B, the transceivers210 may, in one embodiment, generate and transmit positioning signals215. The positioning signals 215 may be received by one or more of thesensors 125. Virtually any positioning signal 215 now known or known inthe future to the art may be used. In one embodiment, the positioningsignals 215 are spread-spectrum sequences. For example, the positioningsignals 215 may be orthogonal sequences, such as Kasami sequences,Maximal sequences, and the like. However, it will be appreciated thatthe functions of generating and transmitting need not be embodied in asingle device. In alterative embodiments, the transceivers 210 mayinclude independent devices (not shown) for generating and transmitting.The streamer cables 205 may be deployed by any desirable meansincluding, but not limited to, spooling the streamer cables 205 from thestem of the survey vessel 105. Although not necessary for the practiceof the present invention, velocity meters 122 may be coupled to thetransceivers 210.

During and after deployment of the streamer cables 205, the size andshape of the streamer cable 205, currents in the body of water 117, thevelocity of the survey vessel 105, and other like factors may cause thestreamer cable 205 to move unpredictably through the water, as shown inFIGS. 2C-D. The accuracy of measurements made by various devices (notshown) attached to the streamer cables 205 may, however, depend upon anaccurate knowledge of the relative position of the plurality of thestreamer cables 205 and/or the absolute position of the streamer cables205. Thus, the generated and received positioning signal 215 may becommunicated to the signal processing unit 150. For example, thetransceivers 210 and the sensors 125 may communicate the generated andreceived positioning signals 215, respectively, to the signal processingunit 150 via a data telemetry unit (not shown) included in thetransceivers 210 and/or the sensors 125.

The signal processing unit 150 may determine the relative and/orabsolute locations of the streamer cables 205 by cross-correlating thegenerated and received positioning signals 215. Although the signalprocessing unit 150 depicted in FIGS. 2A-D is positioned on the surveyvessel 105, the present invention is not so limited. In alternativeembodiments, portions of the signal processing unit 150 may bepositioned in the transceivers 210, the sensors 125, the streamer cables205, on the survey vessel 105, or at any other desirable locationwithout departing from the scope of the present invention.

As described in more detail below, computed Doppler-shifted positioningsignals (not shown) corresponding the positioning signals 215 are alsoprovided, in accordance with the second embodiment of the presentinvention. The computed Doppler-shifted positioning signals may then becorrelated with the received positioning signal and/or the generatedpositioning signal 215 to determine the absolute and/or relativepositions of the transceivers 210 and the sensors 125 on the seismiccables 205. Consequently, the present invention may be used fordetermining a propagation time delay between the various transceivers210 and sensors 125. Thus, it may be possible to determine the absoluteand/or relative positions of the seismic cables 205 in situationswherein movement of the survey vessel 105, the cables 205, the sensors125 and/or the transceivers 210 Doppler shifts the frequencies of thereceived signal and/or the positioning signal 215.

The source 110 and/or the transceivers 210 shown in FIGS. 2A-D maygenerate a plurality of positioning signals 130, 215. In one embodiment,the plurality of positioning signals 130, 215 are separable. The term“separable,” as used hereinafter, will be understood to mean that thecross-correlation of a first separable positioning signal 130, 215 witha second separable positioning signal 130, 215 is negligible. In oneembodiment, the positioning signals 130, 215 are orthogonalspread-spectrum sequences, which may be transmitted and/or receivedwhile other positioning signals 130, 215 are also being transmittedand/or received. In particular, the plurality of positioning signals130, 215 may be transmitted and/or received simultaneously.

In one embodiment, a Maximal sequence may be used to form the orthogonalpositioning signal 130, 215. The Maximal sequence may include aplurality of elements and each element in the Maximal sequence may be a+1 or a −1, i.e. the Maximal sequence is a binary sequence. By selectingan appropriate series of elements, each generated Maximal sequence maybe used as the orthogonal positioning signal 130, 215 in a mannerwell-known in the art having the benefit of this disclosure. However, inalternative embodiments, any desirable sequence, such as a Kasamisequence, may be used as the orthogonal positioning signals 130, 215without departing from the scope of the present invention.

Referring now to FIG. 3, an exemplary binary sequence 300 is shown. Thebinary sequence 300 varies from +1 to −1 with time and can be Dopplertransformed to form the Doppler-shifted sequence 310. Sequences ingeneral, and Maximal and Kasami sequences in particular, are notnecessarily Doppler invariant, as will be appreciated by those ofordinary skill in the art having the benefit of this disclosure.Consequently, the Doppler-shifted sequence 310 may not be a binarysequence. As illustrated in FIG. 3, the Doppler-shifted sequence 310takes on amplitude values that are not always equal to +1 or −1.

In one embodiment, the Doppler-shifted sequence 310 is converted into abinary Doppler-shifted sequence 320 using a threshold that is set equalto zero. When the amplitude of the Doppler-shifted sequence 310 isgreater than the threshold value of zero, the binary Doppler-shiftedsequence 320 is set to +1. When the amplitude of the Doppler-shiftedsequence 310 is less than the threshold value of zero, the binaryDoppler-shifted sequence 320 is set to −1. The binary Doppler-shiftedsequence 320 formed using the threshold may be correlated with thepositioning signals 130, 215 to determine the position of the sensors125 and/or transceivers 210 on the seismic cable 115. However, it willbe appreciated that the value of the threshold is not material to thepresent invention and any desirable threshold may be used withoutdeparting from the scope of the present invention.

FIG. 4 shows a method of acoustic ranging, in accordance with oneembodiment of the present invention. The source 110 and/or transceivers210 generate (at 400) the positioning signal 130, 215. In oneembodiment, the positioning signals 130, 215 are orthogonal sequences.For example, as discussed above, the positioning signal 130, 215 may bea Maximal sequence, a Kasami sequence, and the like. The positioningsignals 130, 215 then propagate (at 410) to the sensors 125 and/or thetransceivers 210.

The sensors 125 receive (at 420) the positioning signals 130, 215. Thereceived positioning signals 130, 215 are then provided to the signalprocessing unit 150, which correlates (at 430) the generated andreceived positioning signals 130, 215. In one embodiment, the signalprocessing unit 150 cross-correlates (at 430) the generated and receivedpositioning signals 130, 210. The signal processing unit 150 thendetermines (at 440) whether the correlation is reliable. For example, asdiscussed above, the Doppler shift of the generated and receivedpositioning signals 130, 215 may make the cross-correlation of thegenerated and received positioning signals 130, 215 unreliable. If thecorrelation is unreliable, the signal processing unit 150 correlates (at450) the received positioning signals 130, 215 with one or more computedDoppler-shifted positioning signals and then determines (at 440) whetherthe correlation is reliable. In one embodiment the reliability of thecorrelation may be determined using an amplitude threshold applied tothe main peak of the correlation estimate. In an alternative embodiment,the reliability of the correlation may be determined using apeak-to-side-lobe ratio.

Once the received positioning signals 130, 215 have been reliablycorrelated (at 450) with one or more computed Doppler-shiftedpositioning signals, the signal processing 150 unit uses the correlatedsignals to determine (at 460) a propagation time delay between thesource 110 or the transceiver 210 and the sensors 125.

Referring now to FIG. 5, a first method of correlating (at 450) thereceived positioning signals 130, 215 with one or more computedDoppler-shifted positioning signals is illustrated. In the first method,a plurality of velocities is selected (at 500). In one embodiment,selecting (at 500) the plurality of velocities includes selecting arange of velocities and a velocity resolution. For example, the selectedvelocities may range from 4 meters/second to −4 meters/second and thevelocity resolution may be 1 meter/second. However, the presentinvention is not so limited. Any desirable process of selecting thevelocities may be used, including manually selecting velocities, using avelocity dependent resolution, adaptively varying the velocityresolution and/or range to account for changing conditions, and thelike.

A plurality of positioning signals 130, 215 are selected (at 510) suchthat at least one of the selected positioning signals 130, 215 willcorrespond to the positioning signal generated (at 400) by the source110 and/or transceivers 210. The computed Doppler-shifted positioningsignals are computed (at 520) using the plurality of velocities and theselected positioning signals 130, 215. For example, if 10 velocities areselected and 10 positioning signals are selected, then 100 computedDoppler-shifted positioning signals are formed. In one embodiment,computing (at 520) the Doppler-shifted positioning signals 130, 215 mayalso include forming binary computed Doppler-shifted positioningsignals, as described above.

Referring now to FIG. 6, the plurality of computed Doppler-shiftedpositioning signals is then used to form (at 530) a library 600, inaccordance with one aspect of the present invention. In one embodiment,forming (at 530) the library 600 includes creating an index 605, whichmay include a list 610 of entries 620 and links 630 to the entries 620.In one embodiment, each entry 620 may be grouped into one of a pluralityof sets 625, which correspond to one of the plurality of computedDoppler-shifted positioning signals. For example, if five velocities(V=+2, +1, 0, −1, and −2 meters/second) are selected (at 500) and fourpositioning signals 130, 215 are selected (at 510), then the library 600includes 4 sets 625 comprising 20 entries 620. In various embodiments,the library 600 may be stored in digital form on any of a variety ofcomputer-readable storage media, including compact disks, computer harddisk memory, digital tape, and the like.

Using the library 600, a selected one of the plurality of computedDoppler-shifted positioning signals stored in one of the plurality ofentries 620 may be accessed (at 540) and the selected computedDoppler-shifted positioning signal may be correlated (at 550) with thereceived positioning signal 130, 215, as discussed above.

It will be noted that the various steps described above with regard tothe first method of correlating (at 450) the received positioningsignals 130, 215 with one or more computed Doppler-shifted positioningsignals may be carried out at any desirable time. In one embodiment, thelibrary 600 may be formed (at 530) before the acoustic ranging processis carried out. For example, the library 600 may be formed (at 530)before the survey vessel 105 departs and stored on a storage medium (notshown) that can be carried on the survey vessel 105. The library 600 maythen be accessed (at 540) while a seismic survey is being conducted orafter the seismic survey is complete. In an alternative embodiment, thelibrary 600 may be formed (at 530) during the acoustic ranging process.

Referring now to FIG. 7, a second method of correlating (at 450) thereceived positioning signals 130, 215 with one or more computedDoppler-shifted positioning signals is illustrated As discussed above,in one embodiment, one or more velocity meters 122 are coupled (at 700)to the seismic sources 110, sensors 125, and/or transceivers 210. Thevelocity meters 122 determine (at 710) one or more velocities of theseismic sources 110, sensors 125, and/or transceivers 210. The one ormore velocities are determined (at 710) at substantially simultaneouslywith the generation (at 400) of the positioning signals 130, 215. Inthis context, “substantially simultaneously” means that computing (at720) the Doppler-shifted positioning signals 130, 215 using thedetermined velocities and then correlating (at 730) the receivedpositioning signals 130, 215 and the computed Doppler-shiftedpositioning signals results in a determination of the relative and/orabsolute positions of the seismic sources 110, sensors 125, and/ortransceivers 210. It will therefore be appreciated that the term“substantially simultaneously” may include time delays betweengeneration (at 400) of the positioning signals 130, 215 and determining(at 710) one or more velocities of the seismic sources 110, sensors 125,and/or transceivers 210. The time delays may vary depending on suchconditions as the velocity of the survey vessel 105, the significantwave height, water currents, and the like.

By using the positioning signal 130, 215, or a plurality of positioningsignals 130, 215, in the manner described above, the performance ofacoustic ranging systems, such as the first exemplary system 100 and thesecond exemplary system 200, may be improved. For example, in rough seashaving a significant wave height (SWH) of about 8 meters, which mayaccelerate a buoy or vessel to velocities of at least about 2-3 metersper second, the present invention may allow the relative and/or absolutepositions of the source 110, the sensors 125, and/or the transceivers210 to be determined. Thus, it may be possible to determine the positionof the seismic cables 115, 205 in situations wherein movement of thesurvey vessel 105, the source 110, the cables 115 and 205, the sensors125, and/or the transceivers 210 Doppler shifts the frequencies of thereceived signal and/or the positioning signal 130, 215.

The survey vessel 105 is equipped with a rack-mounted computingapparatus 800, illustrated in FIGS. 8A-B with which at least a portionof signal processing unit 150 (shown in FIGS. 2A-D) is implemented. Thecomputing apparatus 800 includes a processor 805 communicating with somestorage 810 over a bus system 815. The storage 810 may include a harddisk and/or random access memory (“RAM”) and/or removable storage suchas a floppy magnetic disk 817 and an optical disk 820. The storage 810is encoded with a data structure 825 storing the data set acquired asdiscussed above, an operating system 830, user interface software 835,and an application 865. The user interface software 835, in conjunctionwith a display 840, implements a user interface 845. The user interface845 may include peripheral I/O devices such as a key pad or keyboard850, a mouse 855, or a joystick 860. The processor 805 runs under thecontrol of the operating system 830, which may be practically anyoperating system known to the art. The application 865 is invoked by theoperating system 830 upon power up, reset, or both, depending on theimplementation of the operating system 830.

Some portions of the detailed descriptions herein are consequentlypresented in terms of a software implemented process involving symbolicrepresentations of operations on data bits within a memory in acomputing system or a computing device. These descriptions andrepresentations are the means used by those in the art to mosteffectively convey the substance of their work to others skilled in theart The process and operation require physical manipulations of physicalquantities. Usually, though not necessarily, these quantities take theform of electrical, magnetic, or optical signals capable of beingstored, transferred, combined, compared, and otherwise manipulated. Ithas proven convenient at times, principally for reasons of common usage,to refer to these signals as bits, values, elements, symbols,characters, terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated or otherwise as may be apparent, throughout thepresent disclosure, these descriptions refer to the action and processesof an electronic device, that manipulates and transforms datarepresented as physical (electronic, magnetic, or optical) quantitieswithin some electronic device's storage into other data similarlyrepresented as physical quantities within the storage, or intransmission or display devices. Exemplary of the terms denoting such adescription are, without limitation, the terms “processing,”“computing,” “calculating,” “determining,” “displaying,” and the like.

Note also that the software implemented aspects of the invention aretypically encoded on some form of program storage medium or implementedover some type of transmission medium. The program storage medium may bemagnetic (e.g., a floppy disk or a hard drive) or optical (e.g., acompact disk read only memory, or “CD ROM”), and may be read only orrandom access. Similarly, the transmission medium may be twisted wirepairs, coaxial cable, optical fibre, or some other suitable transmissionmedium known to the art. The invention is not limited by these aspectsof any given implementation.

This concludes the detailed description. The particular embodimentsdisclosed above are illustrative only, as the invention may be modifiedand practiced in different but equivalent manners apparent to thoseskilled in the art having the benefit of the teachings herein.Furthermore, no limitations are intended to the details of constructionor design herein shown, other than as described in the claims below. Itis therefore evident that the particular embodiments disclosed above maybe altered or modified and all such variations are considered within thescope and spirit of the invention. Accordingly, the protection soughtherein is as set forth in the claims below.

1. An apparatus for determining a propagation time delay, comprising: atleast one source adapted to generate a plurality of positioning signals;at least one receiver deployed along a seismic sensing cable, whereinthe receiver is adapted to receive the plurality of positioning signalsfrom the at least one source; a plurality of computed Doppler-shiftedpositioning signals corresponding to the plurality of positioningsignals; and a signal processing unit adapted to determine a propagationtime delay between the source and the receiver using the generatedpositioning signals, the received positioning signals, and the pluralityof computed Doppler shifted positioning signals.
 2. The apparatus ofclaim 1, wherein the plurality of computed Doppler shifted positioningsignals are stored in a library.
 3. The apparatus of claim 2, whereinthe library comprises a plurality of sets of computed Doppler-shiftedpositioning signals, each set corresponding to one of the plurality ofpositioning signals.
 4. The apparatus of claim 3, wherein each computedDoppler-shifted positioning signal in each of the plurality of setscomprises a computed Doppler-shifted positioning signal indicative ofone of a plurality of velocities.
 5. The apparatus of claim 4, whereinthe plurality of velocities comprises a range of velocities having avelocity resolution.
 6. The apparatus of claim 1, further comprising atleast one first velocity meter coupled to the at least one source,wherein the plurality of computed Doppler-shifted positioning signalsare generated using the first velocity meter.
 7. The apparatus of claim1, further comprising at least one second velocity meter coupled to theat least one receiver, wherein the plurality of computed Doppler shiftedpositioning signals are generated using the second velocity meter. 8.The apparatus of claim 1, wherein the plurality of positioning signalscomprises a plurality of sequences.
 9. The apparatus of claim 8, whereinthe plurality of sequences comprises a plurality of separable sequences.10. The method of claim 9, wherein the plurality of separable sequencescomprises a plurality of substantially orthogonal sequences.
 11. Themethod of claim 10, wherein the plurality of substantially orthogonalsequences comprises at least one of a plurality of Kasami sequences anda plurality of Maximal sequences.
 12. The apparatus of claim 1, whereinthe signal processing unit is adapted to determine the propagation timedelay by cross-correlating the generated positioning signal and thereceived positioning signal.
 13. The apparatus of claim 1, wherein thesignal processing unit is adapted to determine the propagation timedelay by cross-correlating the received positioning signal with at leastone computed Doppler-shifted positioning signal.
 14. A method fordetermining a propagation time, comprising: generating at least onepositioning signal using at least one source; receiving the at least onepositioning signal with at least one receiver positioned along a seismiccable; providing at least one computed Doppler-shifted positioningsignal corresponding to the at least one positioning signal; anddetermining at least one propagation time delay from the source to thereceiver using the generated positioning signal, the receivedpositioning signal, and the at least one computed Doppler-shiftedpositioning signal.
 15. The method of claim 14, wherein determining theat least one propagation time delay comprises cross-correlating thegenerated positioning signal and the received positioning signal. 16.The method of claim 14, wherein determining the at least one propagationtime delay comprises cross-correlating the received positioning signalwith the at least one computed Doppler-shifted positioning signal. 17.The method of claim 14, wherein providing the at least one computedDoppler-shifted positioning signal comprises: determining at least onevelocity; determining the at least one computed Doppler-shiftedpositioning signal using the at least one generated positioning signaland the at least one velocity; forming a library using the at least onecomputed Doppler-shifted positioning signal; and accessing the library.18. The method of claim 17, wherein forming the library comprisesproviding an index to the at least one computed Doppler-shiftedpositioning signal.
 19. The method of claim 14, further comprisingcoupling a first velocity meter to the source.
 20. The method of claim19, wherein providing the at least one computed Doppler-shiftedpositioning signal comprises: determining at least one velocity usingthe first velocity meter; and determining the at least one computedDoppler-shifted positioning signal for the at least one positioningsignal using the at least one velocity.
 21. The method of claim 14,further comprising coupling a second velocity meter to the at least onereceiver.
 22. The method of claim 21, wherein providing the at least onecomputed Doppler-shifted positioning signal comprises: determining atleast one velocity using the second velocity meter, and determining theat least one computed Doppler-shifted positioning signal for the atleast one positioning signal using the at least one velocity.
 23. Themethod of claim 14, wherein generating the at least one positioningsignal comprises generating at least one sequence, receiving the atleast one positioning signal comprises receiving at least one sequence,providing the at least one computed Doppler-shifted positioning signalcorresponding to the at least one positioning signal comprises providingat least one computed Doppler-shifted sequence corresponding to the atleast one sequence, and determining the at least one propagation timedelay from the source to the receiver using the generated positioningsignal, the received positioning signal, and the at least one computedDoppler-shifted positioning signal comprises determining the at leastone propagation time delay from the source to the receiver using thegenerated sequence, the received sequence, and the at least one computedDoppler-shifted sequence.
 24. A method for forming a library,comprising: determining a plurality of velocities; selecting a pluralityof positioning signals; determining a plurality of computedDoppler-shifted positioning signals for each of the plurality ofpositioning signals using the plurality of velocities; and providing anindex to the plurality of computed Doppler-shifted positioning signals.25. The method of claim 24, wherein determining the plurality ofvelocities comprises selecting a velocity range.
 26. The method of claim25, wherein selecting the velocity range comprises selecting thevelocity range extending from about 4 meters/second to about −4meters/second.
 27. The method of claim 24, wherein determining theplurality of velocities comprises selecting a velocity resolution. 28.The method of claim 27, wherein selecting the velocity resolutioncomprises selecting the velocity resolution of 1 meter/second.
 29. Themethod of claim 24, wherein selecting the plurality of positioningsignals comprises selecting one positioning signal for each of acorresponding plurality of seismic sources and seismic receivers. 30.The method of claim 24, wherein selecting the plurality of positioningsignals comprises selecting a plurality of sequences.
 31. The method ofclaim 30, wherein selecting the plurality of sequences comprisesselecting a plurality of separable sequences.
 32. The method of claim31, wherein selecting the plurality of separable sequences comprisesselecting a plurality of substantially orthogonal sequences.
 33. Themethod of claim 32, wherein selecting the plurality of substantiallyorthogonal sequences comprises selecting at least one of a plurality ofKasami sequences and a plurality of Maximal sequences.
 34. The method ofclaim 30, wherein determining the plurality of computed Doppler-shiftedpositioning signals comprises determining a plurality of computedDoppler-shifted sequences.
 35. The method of claim 34, whereindetermining the plurality of computed Doppler-shifted sequencescomprises determining a plurality of binary computed Doppler-shiftedsequences.
 36. The method of claim 35, wherein determining the pluralityof binary computed Doppler-shifted sequences comprises determining theplurality of binary computed Doppler-shifted sequences using athreshold.
 37. The method of claim 36, wherein determining the pluralityof binary computed Doppler-shifted sequences using the thresholdcomprises transforming a value of the computed Doppler-shifted sequenceto +1 or −1 using the threshold.
 38. The method of claim 24, furthercomprising storing the library.
 39. A library comprising a datastructure encoded on a computer-readable storage medium, wherein thelibrary comprises: a plurality of computed Doppler-shifted positioningsignals formed by: determining a plurality of velocities; selecting aplurality of positioning signals; and determining the plurality ofcomputed Doppler-shifted positioning signals for each of the pluralityof positioning signals using the plurality of velocities; and an indexof the plurality of computed Doppler-shifted positioning signals. 40.The library of claim 39, wherein the positioning signals are sequences.41. The library of claim 40, wherein the sequences are orthogonalsequences.
 42. The library of claim 41, wherein the orthogonal sequencesare at least one of a Kasami sequence and a Maximal sequence.